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Showing papers in "Heat and Mass Transfer in 2003"


Journal ArticleDOI
TL;DR: In this article, an apparently paradoxical behaviour of heat transfer deterioration was observed in nano-fluid and its dependence on parameters such as particle concentration, material of the particles and geometry of the containing cavity have been investigated.
Abstract: Fluids with nano size solid particles suspended in them have been given the name nano-fluid which in recent studies have shown tremendous promise as heat transfer fluids. However, before suggesting such fluids for applications a thorough knowledge of physical mechanism of heat transfer in such fluids is wanted. The present study deals with one such aspect of natural convection of nano fluids inside horizontal cylinder heated from one end and cooled from the other. An apparently paradoxical behaviour of heat transfer deterioration was observed in the experimental study. Nature of this deterioration and its dependence on parameters such as particle concentration, material of the particles and geometry of the containing cavity have been investigated. The fluid shows characters distinct from that of common slurries.

906 citations


Journal ArticleDOI
TL;DR: In this paper, the authors looked at injection processes in the supercritical regime using cryogenic nitrogen and compared density and divergence angles from 2D Raman images with computational models, providing much information about the jet development and mixing with the surrounding gas.
Abstract: Understanding the complex environment of rocket chambers involves a good knowledge of injection phenomena and gives the designer the ability to employ time and cost saving modeling tools to design a higher performance engine. This project looked at injection processes in the supercritical regime using cryogenic nitrogen. Experimental data taken by 2-D Raman imaging allowed the comparison of density and divergence angels with computational models. These parameters provide much information about the jet development and mixing with the surrounding gas. The process used to derive divergence angles from Raman images proves difficult to compare directly with other techniques.

176 citations


Journal ArticleDOI
TL;DR: In this paper, the steady-state, hydromagnetic forced convective boundary-layer flow of an incompressible Newtonian, electrically-conducting and heat-generating/absorbing fluid over a non-isothermal wedge in the presence of thermal radiation effects is considered.
Abstract: This work is focused on the steady-state, hydromagnetic forced convective boundary-layer flow of an incompressible Newtonian, electrically-conducting and heat-generating/absorbing fluid over a non-isothermal wedge in the presence of thermal radiation effects. The wedge surface is assumed permeable so as to allow for possible wall suction or injection. Also included in the model are the effects of viscous dissipation, Joule heating and stress work. The governing partial differential equations for this investigation are derived and transformed using a non-similarity transformation. In deriving the governing equations, a temperature-dependent heat source or sink term is employed and the Rossland approximation for the thermal radiation term is assumed to be valid. The obtained non-similar equations are solved numerically by an implicit, iterative, tri-diagonal finite-difference method. Comparisons with previously published work on various special cases of the problem are performed and the results are found to be in excellent agreement. Numerical results for the velocity and temperature profiles for a prescribed magnetic parameter as well as the development of the local skin-friction coefficient and local Nusselt number with the magnetic parameter are presented graphically and discussed. This is done in order to elucidate the influence of the various parameters involved in the problem on the solution.

140 citations


Journal ArticleDOI
TL;DR: In this paper, the problem of momentum and heat transfer in a thin liquid film of power-law fluid on an unsteady stretching surface has been studied, and numerical solutions are obtained for some representative values of the unsteadiness parameter S and the power law index n for a wide range of the generalized Prandtl number.
Abstract: In this paper the problem of momentum and heat transfer in a thin liquid film of power-law fluid on an unsteady stretching surface has been studied. Numerical solutions are obtained for some representative values of the unsteadiness parameter S and the power-law index n for a wide range of the generalized Prandtl number, 0.001 ≤ Pr ≤ 1000. Typical temperature and velocity profiles, the dimensionless film thickness, free-surface temperature, and the surface heat fluxes are presented at selected controlling parameters. The results show that increasing the value of n tends to increase the boundary-layer thickness and broadens the temperature distributions. The free-surface temperature of a shear thinning fluid is larger than that of a Newtonian fluid, but the opposite trend is true for a shear thickening fluid. For small generalized Prandtl numbers, the surface heat flux increases with a decrease in n, but the impacts of n on the heat transfer diminish for Pr greater than a moderate value (approximately 1 ≤ Pr ≤ 10, depending on the magnitude of S).

137 citations


Journal ArticleDOI
TL;DR: In this article, a model of heat and moisture transport in firefighter protective clothing during a flash fire exposure is presented, and the duration of the exposure during which the garment protects the firefighter from getting second and third degree burns from the flash-fire exposure is numerically predicted.
Abstract: In this paper, a model of heat and moisture transport in firefighter protective clothing during a flash fire exposure is presented. The aim of this study is to investigate the effect of coupled heat and moisture transport on the protective performance of the garment. Computational results show the distribution of temperature and moisture content in the fabric during the exposure to the flash fire as well as during the cool-down period. Moreover, the duration of the exposure during which the garment protects the firefighter from getting second and third degree burns from the flash fire exposure is numerically predicted. A complete model for the fire-fabric-air gap-skin system is presented.

113 citations


Journal ArticleDOI
TL;DR: In this paper, an analysis was developed to study the unsteady mixed convection flow over a vertical cone rotating in an ambient fluid with a time-dependent angular velocity in the presence of a magnetic field, and the coupled nonlinear partial differential equations governing the flow were solved numerically using an implicit finite-difference scheme.
Abstract: An analysis is developed to study the unsteady mixed convection flow over a vertical cone rotating in an ambient fluid with a time-dependent angular velocity in the presence of a magnetic field. The coupled nonlinear partial differential equations governing the flow have been solved numerically using an implicit finite-difference scheme. The local skin friction coefficients in the tangential and azimuthal directions and the local Nusselt number increase with the time when the angular velocity of the cone increases, but the reverse trend is observed for decreasing angular velocity. However, these are not mirror reflection of each other. The magnetic field reduces the skin friction coefficient in the tangential direction and also the Nusselt number, but it increases the skin friction coefficient in the azimuthal direction. The skin friction coefficients and the Nusselt number increase with the buoyancy force.

112 citations


Journal ArticleDOI
TL;DR: In this paper, a simplified analytical model based on a quasi-linear, transient, thin-fin equation was proposed to predict the solid-liquid interface location and temperature distribution of the fin in the melting process with a constant imposed end-wall temperature.
Abstract: The most PCMs with high energy storage density have an unacceptably low heat conductivity and hence internal heat transfer enhancement techniques such as fins or other metal structures are required in latent heat thermal storage (LHTS) applications. Previous work has concentrated on numerical and experimental examination in determining the influence of the fins in melting phase change material. This paper presents a simplified analytical model based on a quasi-linear, transient, thin-fin equation which predicts the solid–liquid interface location and temperature distribution of the fin in the melting process with a constant imposed end-wall temperature. The analytical results are compared to the numerical results and they show good agreement. Due to the assumptions made in the model, the speed of the solid–liquid interface during the melting process is slightly too slow.

107 citations


Journal ArticleDOI
TL;DR: In this paper, the authors derived several closed form analytical solutions for non-dimensional temperature, concentration, heat flux, mass flux profiles in the form of confluent hyper geometric (Kummer's) functions and some other elementary functions as its special form, for two different cases of the boundary conditions, namely, (i) wall with prescribed second order power law temperature (PST) and prescribed second-order power law concentration (ii) wall in PHF), in comparison with the PST case when the wall is maintained with prescribed power law surface concentration.
Abstract: The present paper deals with the study of momentum, heat and mass transfer characteristics in a viso-elastic fluid flow over a porous sheet, where the flow is generated due to linear stretching of the sheet and influenced by a uniform magnetic field applied vertically and a continuous injection of the fluid through porous boundary. In the flow region, heat balance is maintained with a temperature dependent heat source/sink, viscous dissipation, dissipation due to elastic deformation and stress work produced as the result of magnetic field on the non-Newtonian fluid. In mass transfer analysis we have taken into account the loss of mass of the chemically reactive diffusive species by means of first order chemical conversion rate. Using suitable similarity transformations on the highly non-linear partial differential equations we derive several closed form analytical solutions for non-dimensional temperature, concentration, heat flux, mass flux profiles in the form of confluent hyper geometric (Kummer's) functions and some other elementary functions as its special form, for two different cases of the boundary conditions, namely, (i) wall with prescribed second order power law temperature (PST) and prescribed second order power law concentration (ii) wall with prescribed second order power law heat flux (PHF) and prescribed second order power law mass flux. The effect of the non-dimensional magnetic parameter on momentum, heat and mass transfer characteristics for non-isothermal boundary condition and different physical situations of the fluid, having various degrees of visco-elasticity, Prandtl number, heat source/sink strength and Schmidt number, are discussed in detail. Some of the several important findings reported in this paper are: (i) The combined effect of magnetic field, visco-elasticity and impermeability of the wall is to increase skin-friction largely at the wall; (ii) maximum enhancement of wall-temperature profile due to the application of transverse magnetic field occurs when the boundary heating is maintained with prescribed heat flux, boundary wall is porous and Prandtl number of the fluid is low; (iii) the effect of transverse uniform magnetic field is to increase concentration in the flow region, however, enhancement of concentration is higher when the stretching wall is porous and subjected to injection and (iv) the reduction of concentration due to chemical conversion is of significant order near the wall in PHF case when the wall is maintained with prescribed power low mass flux, in comparison with the PST case when the wall is maintained with prescribed power law surface concentration.

88 citations


Journal ArticleDOI
M. Abu-Zaid1
TL;DR: In this article, the authors investigated the effect of water in diesel and kerosene emulsions on the evaporation time of a single droplet, on hot surfaces (stainless-steel and aluminum).
Abstract: This paper presents the results of an experimental investigation, into the effect of water in diesel and kerosene emulsions, on the evaporation time of a single droplet, on hot surfaces (stainless-steel and aluminum). Experiments are performed at atmospheric pressure, and initial water volume concentrations of 10, 20, 30, and 40%. The wall temperatures ranging from 100–460 °C, to cover the entire spectrum of heat transfer characteristics from evaporation to film boiling. Results show that, qualitatively, the shapes of emulsion evaporation curves are very similar to that of pure liquids. Quantitavely, there are significant differences. The total evaporation time, for the emulsion droplets is lower than that for diesel and kerosene fuels, and decreased as water initial concentration increases, up to surface temperatures less than the critical temperature. The value of the critical surface temperature (maximum heat transfer rate), decreases as initial concentration of water increases. In the film-boiling region, the evaporation time for the emulsion droplets is higher than for diesel and kerosene droplets, at identical conditions.

68 citations


Journal ArticleDOI
TL;DR: In this paper, the authors analyzed the convective heat transfer enhancement mechanism of microencapsulated phase change material slurries based on the analogy between heat transfer and thermal conduction with thermal sources.
Abstract: This paper analyzes the convective heat transfer enhancement mechanism of microencapsulated phase change material slurries based on the analogy between convective heat transfer and thermal conduction with thermal sources The influence of each factor affecting the heat transfer enhancement for laminar flow in a circular tube with constant wall temperature is analyzed using an effective specific heat capacity model The model is validated with results available in the literature The analysis and the results clarify the heat transfer enhancement mechanism and the main factors influencing the heat transfer In addition, the conventional Nusselt number definition of phase change slurries for internal flow is modified to describe the degree of heat transfer enhancement of microencapsulated phase change material slurries The modification is also consistent evaluation of the convective heat transfer of internal and external flows

64 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of Rayleigh numbers, number of partition and height of partition on fluid flow and heat transfer in partially divided square enclosures is numerically investigated. But, the authors assume that the horizontal end walls of enclosures are adiabatic or perfectly conducting and the left side wall and the right side wall are maintained at uniform but different temperature.
Abstract: In this study, fluid flow and heat transfer in partially divided square enclosures are numerically investigated. It is assumed that, the horizontal end walls of enclosures are adiabatic or perfectly conducting and the left side wall and the right side wall of enclosures are maintained at uniform but different temperature. Temperature and velocity distributions in the enclosure for laminar and steady flow are determined by solving continuity, momentum and energy equations for appropriate boundary conditions. A computer program based on the control volume approach and SIMPLE algorithm is developed. A conjugate formulation at solid-fluid interface is used and thermal conductivity and viscosity are calculated using a harmonic mean formulation in order to handle abrubt changes in the material properties. By using the fact that, heat transfer from hot side wall by conduction must be equal to the heat transfer to the fluid by convection, the local Nusselt number is evaluated. The effect of Rayleigh numbers, number of partition and height of partition on fluid structure and fluid flow are investigated. It is observed that, mean Nusselt number increases with increasing Rayleigh number and decreases with increasing number of partition, however, decrease in mean Nusselt number is less at low Rayleigh numbers. Furthermore, increasing partition height, decreases mean Nusselt number.

Journal ArticleDOI
TL;DR: In this article, a finite difference code based on vorticity vector potential approach is developed to solve the governing equations and the steady behavior of flow and temperature distributions is obtained from the solution of transient form of governing equations.
Abstract: Steady state natural convection heat transfer in a longitudinally short rectangular fin array on a horizontal base is numerically investigated The problem is three dimensional laminar natural convection heat transfer with open boundaries A finite difference code based on vorticity-vector potential approach is developed to solve the governing equations The steady behaviour of the flow and temperature distributions is obtained from the solution of transient form of the governing equations Results are compared with the available experimental results in the literature A good agreement is seen between the numerical and reported experimental results Flow configurations occurring in the channel of the fin arrays with different geometrical parameters, (fin length, fin height and fin spacing) are analysed Based on the obtained results, two types of flow patterns are observed The mechanisms of the flows are discussed and flow patterns are plotted to support the discussion The study is limited to Rayleigh number based on fin spacing, ranging from 120 to 39000 The fin length and fin height are varied from 2 to 20 and 025 to 7-fin spacing, respectively

Journal ArticleDOI
TL;DR: In this paper, a closed form solution for the temperature distributions inside the solid substrate due to convective boundary condition at the surface is presented, particularly for drying applications, where the analytical solution for diffusion equation is introduced with constant concentration at surface case.
Abstract: In the present study, a closed form solution for the temperature distributions inside the solid substrate due to convective boundary condition at the surface is presented, particularly for drying applications. The analytical solution for the diffusion equation is introduced with constant concentration at the surface case. Temperature and moisture distributions inside apple slab are computed in this regard. It is found that temperature rises rapidly in the surface region of the substrate material during the early heating period and as the heating period progresses, temperature gradient attains almost steady value with advancing time. Moisture content variation in the surface region is considerably high in the early period and as time progresses, the rate of change of concentration in the substrate reduces. The present model is verified with actual data for heat conduction and moisture diffusion and a considerably high agreement is found.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the effects of inhomogeneity and magnetic field on an isotropic elastic cylinder in a primary magenetic field when the curved surface of the cylinder subject to certain boundary conditions.
Abstract: The present paper concerns the investigation of the stress, temperature and magnetic fields in an isotropic elastic cylinder in a primary magenetic field when the curved surface of the cylinder subject to certain boundary conditions.The system of fundamental equations is solved by means of a finite difference method and the numerical calculations are carried out for the temperature, the components of displacement and the components of stresses with time and through the thickness of the cylinder. The results indicate that the effects of inhomogeneity and magnetic field are very pronounced.

Journal ArticleDOI
TL;DR: In this article, the effect of the magnetic field and Hall current on the unsteady free convection flow over an infinite vertical porous plate, which moves with time-dependent velocity in an ambient fluid, has been studied.
Abstract: The unsteady free convection flow over an infinite vertical porous plate, which moves with time-dependent velocity in an ambient fluid, has been studied. The effects of the magnetic field and Hall current are included in the analysis. The buoyancy forces arise due to both the thermal and mass diffusion. The partial differential equations governing the flow have been solved numerically using both the implicit finite difference scheme and the difference-differential method. For the steady case, analytical solutions have also been obtained. The effect of time variation on the skin friction, heat transfer and mass transfer is very significant. Suction increases the skin friction coefficient in the primary flow, and also the Nusselt and Sherwood numbers, but the skin friction coefficient in the secondary flow is reduced. The effect of injection is opposite to that of suction. The buoyancy force, injection and the Hall parameter induce an overshoot in the velocity profiles in the primary flow which changes the velocity gradient from a negative to a positive value, but the magnetic field and suction reduce this velocity overshoot.

Journal ArticleDOI
TL;DR: In this article, the authors studied the two dimensional flow of an electrically conducting fluid which is an optically thin gray gas past a stationary vertical infinite plate in the presence of radiation.
Abstract: This paper studies the two dimensional flow of an electrically conducting fluid which is an optically thin gray gas past a stationary vertical infinite plate in the presence of radiation. It is assumed that the temperature of the plate and the suction at the plate are constant. The presence of the induced magnetic field is also taken into account. Numerical solutions for the velocity and the induced magnetic field are derived and the effects of the radiation parameter are discussed.

Journal ArticleDOI
P. Ganesan1, G. Palani1
TL;DR: In this article, a numerical study of transient natural convection flow past an impulsively started inclined plate Crank-Nicolson implicit finite difference method is used to solve the unsteady, non-linear and coupled governing equations in order to check the accuracy of the numerical results, the present study is compared with available exact solution and are found to be in good agreement.
Abstract: The present investigation, involving the simultaneous heat and mass transfer is concerned with a numerical study of transient natural convection flow past an impulsively started inclined plate Crank-Nicolson implicit finite difference method is used to solve the unsteady, non-linear and coupled governing equations In order to check the accuracy of the numerical results, the present study is compared with available exact solution and are found to be in good agreement Numerical results are obtained for various parameters The steady-state velocity, temperature and concentration profiles, local and average skin friction, local and average Nusselt number, local and average Sherwood number are shown graphically It is observed that local wall shear stress decreases as an angle of inclination φ decreases

Journal ArticleDOI
TL;DR: In this article, the effect of surface waviness, determined by wavelength-amplitude ratio λ/a (=L*) on flow and thermal field, was investigated numerically in a pipe with sinusoidal wavy surface.
Abstract: Hydrodynamic and thermal characteristics in a pipe with sinusoidal wavy surface for steady laminar flow are investigated numerically in the present study. Governing equations are discretized using control volume based Finite-Volume method with collocated variable arrangement. SIMPLE algorithm is used and TDMA solver is applied for solution of system of equations. A pipe of length equal to 4λ is considered where λ is the wavelength. Effect of surface waviness, determined by wavelength-amplitude ratio λ/a (=L*) on flow and thermal field is presented. The simulation work has been carried out for L*=11.0 – 30.0 for a Reynolds number ranging from 50 to 2000. Results are presented in the form of streamfunction, isothermal lines, velocity and vorticity profiles, Nusselt number with Reynolds number, wave number (n) and L*. For a particular geometry, length of circulation zone increases with Reynolds number and approaches a limiting value. Wall shear stress is smaller in the bulge part of the channel that affects largely the rate of heat transfer. These circulation zone increases with the increase of surface waviness showing high mean friction factor per wavelength. Mean friction factor inversely varies with Reynolds number. A correlation is proposed for calculating the friction factor in the form f=C/(Re)m, where 'C' and 'm' represents two polynomials of degree 4 and 2 respectively and are function of L*.

Journal ArticleDOI
TL;DR: In this paper, the authors used the single probe and dual probe methods to measure and compare thermal conductivities of two soils, namely, sand and loam, and found that sand had higher values of thermal conductivity than loam.
Abstract: The thermal conductivity of two soils was investigated through laboratory studies. These laboratory experiments used the single probe and dual probe methods to measure and compare thermal conductivities. The soils used were classified as sand and loam. Thermal conductivity measured using single probe method ranged from 0.95 to 2.11 for sand and from 0.49 to 0.76 W/m K for loam. Thermal conductivity measured using dual probe method ranged from 0.98 to 2.17 for sand and from 0.51 to 0.78 W/m K for loam. Finally, it was found that sand had higher values of thermal conductivity than loam for all soil conditions studied.

Journal ArticleDOI
TL;DR: In this article, the authors used the second order schemes for the space and time discretizations to obtain the numerical results of heat transfer by natural convection of air in a tall cavity.
Abstract: Transient laminar natural convection of air in a tall cavity has been studied numerically. The Navier-Stokes and Energy equations were solved by the accurate projection method (PmIII), in which the derived Poisson equation for pressure potential was solved by the approximate factorization one method (AF1). The aspect ratio of the tall cavity is 16, and the Prandtl number of air filled in the tall cavity is 0.71. To obtain the numerical results of heat transfer by natural convection of air in the tall cavity, the second order schemes for the space and time discretizations were utilized. The availability of the numerical algorithm was also assessed by considering the natural convection of air in a square cavity which is differentially heated from side walls. It was found that the overall Nusselt numbers for the Rayleigh numbers covering the range from 1000 to 100000 reveal a good agreement with measured data. When Ra takes the value in the range from 100000 to 600000, the overall Nusselt number have a relative deviation less than 18% from the experimental data. For the suddenly heating mode, the multicellular flow pattern occurs when Rayleigh number belongs to the range of Ra from 7000 to 20000. or greater than 115000. At the critical number of cats' eye instability, the cell distance is just twice of the cavity width. This is rather similar to the observed result for Benard problem. When Ra is over 115000, a further increase of heat flux across the tall cavity causes serious cell-breaking. There are 8 cells when Ra = 600000.

Journal ArticleDOI
TL;DR: In this article, an axisymmetric sinusoidal wavy-walled tube is investigated experimentally in the Reynolds number range from 50 to 1000 and the flow patterns are visualized by the aluminum dust method, and the measurements of the wall shear stress and the mass transfer rate for high Schmidt number are performed by the electrochemical method.
Abstract: Flow and mass transfer characteristics in an axisymmetric sinusoidal wavy-walled tube are investigated experimentally in the Reynolds number range from 50 to 1000 The flow patterns are visualized by the aluminum dust method The measurements of the wall shear stress and the mass transfer rate for high Schmidt number are performed by the electrochemical method The attention is focused on their characteristics, including the transitional flow It was observed that steady flow changes into unsteady flow when the Reynolds number exceeds about 160 The wall shear stress in the minimum circular cross section of the tube and the mass transfer rate for one wavelength are shown to depend upon the Reynolds number In the laminar flow regime they increase with the slope of 1 and 1/3, respectively, whereas in the turbulent flow regime they increase with the slope of 3/2 and 3/5, respectively In the transitional flow regime both of them increase significantly with further larger slope It is found that the laminar-like motion and turbulent-like motion occur alternatively, with different time intervals, indicating intermittent flow behavior This is quite different from the flow instability of the wavy-walled channel in which Tollmien-Schlichting waves keeping a time-periodic flow appear

Journal ArticleDOI
TL;DR: In this article, the authors applied elasto-plastic analysis when modeling the thermal stresses due to a laser heating pulse and found that stress levels higher than the yield stress of the substrate material occur in the surface region.
Abstract: Laser heating of multilayer assembly results in temperature profiles, which differ in each layer. Depending upon the material and laser pulse properties, high temperature gradients can occur. This, in turn, results in excessive stress levels in the region irradiated by a laser beam. When the magnitude of stress level exceeds the yield stress of the material, plastic deformation is resulted. In this case, elasto-plastic analysis should be applied when modeling the thermal stresses due to a laser heating pulse. In the present study, time exponentially varying pulse laser heating of four and two layer assemblies is considered. The four layer assembly composes of gold, chromium, gold and silicon while two layer assembly is formed from gold and silicon. The resulting temperature field is obtained numerically using a control volume approach. The elasto-plastic analysis employed to compute the stress levels in the substrate material. It is found that stress levels higher than the yield stress of the substrate material occurs in the surface region. This, in turn, results in plastic zone in this region. The size of the plastic region extends towards the solid bulk as the heating progresses.

Journal ArticleDOI
TL;DR: In this paper, a test section has been built up simulating a hot plate, which is divided into 8 independently heated modules of 10 mm length to enable local heat transfer measurements, by means of temperature controlled systems for each module local steady state experiments in the whole range between single phase heat transfer and film boiling.
Abstract: Jet impingement boiling is very efficient in cooling of hot surfaces as a part of the impinging liquid evaporates. Because of its importance to many cooling procedures, investigations on basic mechanisms of jet impingement boiling heat transfer are needed. Until now, most of the experimental studies, carried out under steady-state conditions, used a heat flux controlled system and were limited by the critical heat flux (CHF). The present study focuses on steady-state experiments along the entire boiling curve for hot plate temperatures of up to 700°C. A test section has been built up simulating a hot plate. It is divided into 8 independently heated modules of 10 mm length to enable local heat transfer measurements. By means of temperature controlled systems for each module local steady-state experiments in the whole range between single phase heat transfer and film boiling are possible. By solving the two dimensional inverse heat conduction problem, the local heat flux and the corresponding wall temperature on the surface of each module can be computed. The measurements show important differences between boiling curves measured at the stagnation line and those obtained in the parallel flow region. At the stagnation line, the transition boiling regime is characterised by very high heat fluxes, extended to large wall superheats. Inversely, boiling curves in the parallel flow region are very near to classical ones obtained for forced convection boiling. The analysis of temperature fluctuations measured at a depth of 0.8 mm from the boiling surface enables some conclusions on the boiling mechanism in the different boiling regimes.

Journal ArticleDOI
TL;DR: In this article, an experimental study has been carried out to measure boiling heat transfer coefficient of an ammonia/water mixture on a horizontal heated surface at low pressure of 0.27 and at high pressure 0.7 MPa and at low mass fraction of 0 < C < 0.
Abstract: The nonazeotropic binary mixtures such as, methanol/water, ethanol/water and ammonia/water, have variable boiling and dew points, depending on the combination of substance and those mass fraction. It is expected to have a higher performance as a result of decreasing the thermodynamically irreversible loss, when there is a relevant mass fraction. Therefore, ammonia/water mixture is expected to use as working fluid in small temperature difference power generation cycles and absorption refrigeration cycles. However, few experiments were carried out for measuring heat transfer coefficient for ammonia/water mixture in the world. An experimental study has been carried out to measure boiling heat transfer coefficient of an ammonia/water mixture on a horizontal heated surface at low pressure of 0.2, 0.4 and 0.7 MPa and at low mass fraction of 0 < C < 0.27 and at high pressure 0.7, 1.0 and 1.5 MPa and at mass fraction of 0.5 < C < 1.0 and at heat flux under critical heat flux the heat transfer coefficient are compared with existing correlations prediction and a revised correlation can be proposed to predict them well.

Journal ArticleDOI
TL;DR: In this article, the thermal convection flow in a fluid-saturated porous cavity with external sidewall heating and internal heat generation is studied by a semi-implicit projection finite element method.
Abstract: In this paper, the thermal convection flow in a fluid-saturated porous cavity with external sidewall heating and internal heat generation is studied by a semi-implicit projection finite element method. Concerned physical phenomena are constructed by impulsively switching on a uniform internal heat generation at t = 0 in a steady state flow field established by two different sidewall temperatures. Flows in the range 103 ≤ Ra I ≤ 108 and Ra E = 105 are mainly investigated for porous media ɛ = 0.4 and 10–10 < Da < 10–1. The results include the steady state flow field and heat transfer phenomena, as well as the evolution of heat transfer. According to the results, porosity demonstrates obvious influence in heat transfer as Da ≥ 10–2, and permeability significantly affects the flow field and heat transfer rate for Da < 10–2. The decrease in permeability diminishes the strength of external heating driven circulation and provides a chance for the cell driven by internal heat generation to develop. In addition, the flow in a porous medium with smaller flow restriction more quickly reaches the steady state from the instantaneously started internal heat generation.

Journal ArticleDOI
TL;DR: In this article, the effect of locally variable heat transfer coefficient on the performance of extended surfaces (fins) subject to natural convection was investigated and compared with the results obtained for the corresponding fin subject to constant heat transfer coefficients along the fin.
Abstract: The present article investigates the effect of locally variable heat transfer coefficient on the performance of extended surfaces (fins) subject to natural convection. Fins of different profiles have been investigated. The fin profiles presently considered are namely; straight and pin fin with rectangular (constant diameter), convex parabolic, triangular (conical) and concave parabolic profiles and radial fins with constant profile with different radius ratios. The local heat transfer coefficient was considered as function of the local temperature and has been obtained using the available correlations of natural convection for each pertinent extended surface considered. The performance of the fin has been expressed in terms of the fin efficiency. Comparisons between the present results for all fins considered and the results obtained for the corresponding fins subject to constant heat transfer coefficient along the fin are presented. Comparisons, i.e. showed an excellent agreement with the experimental results available in the literature. Results show that there is a considerable deviation between the fin efficiency calculated based on constant heat transfer coefficient and that calculated based on variable heat transfer coefficient and this deviation increases with the dimensionless parameter m.

Journal ArticleDOI
TL;DR: In this paper, an analysis is performed to study the unsteady incompressible flow over a moving vertical cylinder with constant heat flux and magnetic field applied normal to the surface.
Abstract: An analysis is performed to study the unsteady incompressible flow over a moving vertical cylinder with constant heat flux and magnetic field applied normal to the surface. The nonlinear-coupled parabolic partial differential equations governing the flow and heat transfer have been solved numerically using a finite difference scheme of Crank-Nicolson type. The numerical results are presented for the velocity, temperature, local and average skin-friction and Nusselt number. It is observed that the presence as well as increase in the magnetic field leads to decrease in the velocity field and rise in thermal boundary thickness.

Journal ArticleDOI
TL;DR: In this article, a computer code was developed for modeling laminar mixed convection heat and mass transfer with the evaporative cooling of liquid film falling along an insulated vertical channel.
Abstract: A computer code was developed for modelling laminar mixed convection heat and mass transfer with the evaporative cooling of liquid film falling along an insulated vertical channel. The numerical method applied solves the coupled governing equations together with the boundary and interfacial conditions. Results are specifically presented for air–water system. Parametric computations were performed to investigate the effects of Reynolds number, inlet liquid temperature and inlet liquid mass flow rate on the liquid film cooling mechanism. The results indicate that the latent heat transfer is a main mechanism for heat removal from the interface. Additionally, significant liquid cooling results for the system with a higher gas flow Reynolds number Re, a lower liquid flow rate Γ0 or a higher inlet liquid temperature T L0.

Journal ArticleDOI
TL;DR: In this paper, a numerical model and technique are described to simulate the turbulent fluid flow over and heat transfer through a model of vehicle windshield defrosting and demisting systems, and the simplified geometry and the dimensions of the numerical model are representative of vehicle system with accurate locations of the nozzles and outlet vents.
Abstract: A numerical model and technique are described to simulate the turbulent fluid flow over and heat transfer through a model of vehicle windshield defrosting and demisting systems. The simplified geometry and the dimensions of the numerical model are representative of vehicle system with accurate locations of the nozzles and outlet vents, including cabin features such as seating and the rear parcel shelf.

Journal ArticleDOI
TL;DR: In this article, an explicit TVD scheme is used for two-dimensional non-Fourier heat conduction problems in the general coordinate system with both convection and radiation boundary conditions.
Abstract: An explicit TVD scheme is used for two-dimensional non-Fourier heat conduction problems in the general coordinate system with both convection and radiation boundary conditions. The hyperbolic heat flux model is used to simulate the non-Fourier heat conduction. Because of the wave nature of hyperbolic equations, characteristics are used to find the unknown value (either heat flux or temperature) on the boundaries. For convective boundary the unknown temperature is calculated explicitly; for radiation boundary Newton's iteration method is applied to find the boundary temperature. Results of numerical examples agree with the physical expectations indicating that the present approach can be used for modeling non-Fourier heat conduction with complex boundary conditions.